FIG. 1 illustrates a network structure of a universal mobile telecommunications system (UMTS).
The UMTS includes a user equipment (UE), a UMTS terrestrial radio access network (UTRAN), and a core network (CN). The UTRAN includes at least one radio network subsystems (RNS), wherein each RNS includes a radio network controller (RNC) and at least one base station (Node B) managed by the RNC. At least one cell exists in one Node B. Also, since the UMTS is a cellular based communication system, handover is performed as a user equipment moves from a current cell, to which the user equipment belongs, to another cell. A base station of a cell to which the user equipment currently belongs will be referred to as a source base station (source eNB), and a base station of a cell to which the user equipment will newly accesses will be referred to as a target base station (target eNB).
In a communication system suggested in a long term evolution (LTE), when handover is performed, user data of all downlinks, which are stored in the source base station, are forwarded to the target base station, and user data of all uplinks, which have been successfully received in the source base station, are transmitted to the core network. All new downlink user data which have reached the core network during a handover procedure are directly transmitted from the core network to the target base station. Afterwards, the target base station transmits all the user data, which have been received from its core network and the source base station, to the user equipment.
However, the aforementioned handover procedure according to the related art may cause waste of radio and wire resources for the following reasons.
First of all, even in case that the source base station has transmitted random downlink user data to the user equipment and the user equipment has normally received corresponding data, if transmission and reception acknowledgement information of the data is not forwarded to the target base station, the target base station determines that the user equipment has not received the data and should transmit the same user data to the user equipment again.
Likewise, even in case that uplink user data transmitted from the user equipment have been successfully received in the source base station, if their transmission and reception acknowledgement information is not forwarded to the user equipment, the user equipment determines that transmission of the user data has been failed and should transmit the same user data to the target base station again.
Such repeated transmission of the same data under the circumstances that radio resources are limited means that an opportunity for transmitting new data is missed. Since this means that the user data cannot be transmitted within a limited time period, a problem occurs in that a transmission rate may seriously be deteriorated.
Secondly, it may be considered that the user equipment moves from one base station to another base station. Considering a data flow of a downlink in an E-UTRAN system, the user data are arrived in aGW in an external internet network, and undergo encoding and header compression therein and then are forwarded to the base station to which the user equipment currently accesses. The RLC of the base station (eNB) reconstructs each user data block suitable for a wireless environment of a transmission timing point and then RLC PDUs to a corresponding user equipment.
At this time, it may be considered that the user equipment is located at the boundary between regions managed by two base stations, i.e., the current user equipment is connected with the base station A and enters the base station B while moving. In this case, the wireless environment between the user equipment and the base station A becomes poorer whereas the wireless environment between the user equipment and the base station B becomes better. This means that the probability that the RLC PDU transmitted from the base station A will not be successfully arrived in the user equipment becomes increased. At this time, the base station A determines that a connection point of the user equipment should be changed to the base station B, and commands the user equipment to connect with the base station B.
In this case, the data blocks which the base station A has not successfully transmitted to the user equipment should be forwarded from the base station A to the base station B, so that the base station B can transmit the data blocks to the user equipment again. However, re-transmission of the data which have been successfully received in the user equipment should be avoided within the limits of the possible. This is because that unnecessarily repeated transmission of the same data may be a waste when considering that possible radio resources are limited. Accordingly, it is necessary to reduce forwarding of the data blocks, which have been successfully received in the user equipment, from the base station A to the base station B within the limits of the possible.